Multiple discharge stock treatment apparatus and method

ABSTRACT

Stock treatment apparatus particularly suited for the bleaching of fibrous material such as pulp by chemicals such as chlorine or the hydrosulfides, comprising a generally upright elongated reaction vessel which is supplied inlet stock at its lower end. The vessel is provided with tangential outlets or stock discharge conduits arranged to discharge treated stock at locations spaced vertically from each other and above the vessel lower end; and above each outlet at least one tangential liquid supply conduit is connected to the vessel for supplying treated stock, extracted chemicals or other liquid thereto. The outlets and liquid supply conduits are provided with valves operable to permit alternative employment of each outlet and its respective vertically adjacent liquid supply conduit such that, by selective operation of the valves, the elevation at which treated stock is discharged from the vessel is variable to vary the period of retention of stock in the vessel. Also, a method for stock treatment through the employment of such an apparatus.

United States Patent Primary Examinerl-loward R. Caine Attorneys-Carl R Horten, David W. Tibbott and Robert R.

Paquin ABSTRACT: Stock treatment apparatus particularly suited for the bleaching of fibrous material such as pulp by chemicals such as chlorine or the hydrosulfides, comprising a generally upright elongated reaction vessel which is supplied inlet stock at its lower end. The vessel is provided with tangential outlets or stock discharge conduits arranged to discharge treated stock at locations spaced vertically from each other and above the vessel lower end; and above each outlet at least one tangential liquid supply conduit is connected to the vessel for supplying treated stock, extracted chemicals or other liquid thereto. The outlets and liquid supply conduits are provided with valves operable to permit alternative employment of each outlet and its respective vertically adjacent liquid supply conduit such that, by selective operation of the valves, the elevation at which treated stock is discharged from the vessel is variable to vary the period of retention of stock in the vessel. Also, a method for stock treatment through the employment of such an apparatus.

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PATENTEnuuv 23 I97l SHEET 2 OF 2 ATTORNEY MULTIPLE DISCHARGE STOCK TREATMENT APPARATUS AND METHOD BACKGROUND OF THE INVENTION The present invention relates to stock treatment apparatus and methods and more specifically to stock treatment apparatus and methods particularly suited for employment in the bleaching of fibrous material such a pulp for the manufacture of paper. 1

In the bleaching of fibrous material such as pulp by oxidizing agents such as chlorine or reducing agents such as hydrosulfides, the initial reaction is relatively rapid but the reaction rate soon declines such that total reaction time is normally minutes or more. The optimum time for the reaction is dependent upon numerous variables such as, for example, the temperatures of the reacted substances and caries for different sets of operating conditions. However, the total reaction time producing the most desirable product for each specific set of operating conditions is constant and predeterminable.

Conventionally, the bleaching of pulp by said chemicals is generally accomplished in an upflow reaction vessel (i.e., a vessel in which the pulp-chemical mixture moves upwardly during its retention) to insure that the mixture is not exposed to the atmosphere until after the reaction has been substantially completed. This isolation of the mixture from the atmosphere is important as, when chlorine is employed, loss of the chemical occurs upon its exposure to the atmosphere and, when hydrosulfides are used, atmospheric oxygen reacts with the chemical. Inherently the nature of the pulp tends to cause short circuiting or uneven flow of the mixture upwardly in the reaction vessel and resultant uneven bleaching or reaction of the pulp. This uneven reaction provides the bleached pulp with portions which are overreacted or overbleached and other portions which are underreacted or underbleached thereby providing a less desirable product and resulting in wasting of the chemical.

Numerous apparatuses and methods have been provided in attempts to obtain successful pulp bleaching with a minimum of short circuiting in the reaction vessel. Generally, however, these conventional apparatuses and methods have attained at the most only partial success and have been unable to permit the period of retention of material in the vessel, and hence the reaction time, to be varied.

U.S. Pat. No. 3,301,027, issued Jan. 31, 1967, to John P. Rich and assigned to the assignee of the present invention, does disclose a pulp-bleaching apparatus which is intended for permitting variable retention of the pulp-chemical mixture in the reaction vessel. However, the apparatus of said patent is dependent for successful operation on a circulator or impeller provided with a single liquid inlet and located at the upper end of the vessel. The corrosive gases filling the upper end of the vessel during the bleaching operation are highly damaging to such an impeller or circulator; and consequently premature impeller failure is an inherent danger in such an apparatus. Also, it is believed that during the operation of such patented apparatus with the vessel containing only a relatively low level of pulp-chemical mixture, the effects of the circulator and liquid fed through the liquid inlet are not sufficiently great to provide a sufficient influence on the mixture.

SUMMARY OF THE INVENTION An object of the present invention is to provide a new and improved stock treatment apparatus which is particularly constructed and arranged to permit the retention time of stock in the vessel to be varied.

Another object of the invention is to provide a new and improved stock treatment apparatus of the type set forth which is particularly constructed and arranged to substantially minimize short circuiting of the stock in the reaction vessel.

Other objects are to provide a new and improved stock treatment apparatus of the type set forth which is particularly suited for employment in the bleaching of fibrous material such as pulp for the manufacture of paper by chemicals such as chlorine or the hydrosulfides, and which is relatively simple and economical in construction and operation.

These objects, and other objects and advantages of the invention which will be apparent from the following description taken in connection with the accompanying drawings, are attained by the provision of stock treatment apparatus in general comprising a generally upright elongated reaction vessel provided with inlet means for supplying inlet stock to one end of the vessel whereupon the inlet stock moves longitudinally through the vessel towards the other end thereof, a plurality of outlets or stock discharge conduits open to the vessel for discharging treated stock from the vessel at locations spaced vertically from each other and spaced vertically from said one end of the vessel, and liquid supply means for supply ing liquid to the vessel at a location vertically adjacent each of the locations at which treated stock is discharged from the vessel by the outlets, the outlets being alternatively operable for discharging treated stock from the vessel and the liquid supply means being operable during discharge of treated stock through each of the outlets for supplying liquid to the vessel at the location vertically adjacent that from which the treated stock is discharged.

Another object of the invention is to provide a new and improved method for stock treatment through the employment of such an apparatus, which method is particularly suited for the bleaching of fibrous material such as pulp by chemicals such as chlorine or the hydrosulfides.

This object is attained by the provision of a method for treating stock in a generally upright elongated reaction vessel provided with plurality of outlets at locations spaced vertically along its length, such method in general comprising the steps of introducing inlet stock to one end of the vessel whereupon the inlet stock moves longitudinally through the vessel towards the other end thereof, and discharging treated stock through the outlets alternatively while simultaneously during the discharge of treated stock through each of the outlets supplying liquid to the vessel at a location vertically adjacent the one of the outlets through which the treated stock is discharged.

BRIEF DESCRIPTION OF THE DRAWINGS Referring to the drawings:

FIG. 1 is an elevational view, partially broken away in section, illustrating one embodiment of an apparatus constructed in accordance with the present invention;

FIG. 2 is a sectional view taken along line 2-2 of FIG. I, looking in the direction of the arrows; and

FIG. 3 is an elevational view illustrating an apparatus constructed in accordance with an alternative embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring more particularly to the drawings wherein similar reference characters designate corresponding parts throughout the several views, FIGS. 1 and 2 illustrate a stock treatment apparatus in the form of a pulp-bleaching apparatus, designated generally as 10, which is particularly intended for chlorine bleaching of pulp. The apparatus 10 comprises an elongated generally upright or vertical cylindrical reaction vessel 12 provided with inlet means which, during the operation of the apparatus 10, continuously introduces the pulp-chemical mixture or inlet stock to the vessel lower end at a controlled volume and rate. The illustrated inlet means comprises an inlet pipe or conduit 14 connected to receive pulpchemical mixture or inlet stock pumped by a pump 22 from a source or reservoir 24, and a stock distributor designated generally as 16 in the lower end of the vessel 12. The stock distributor 16 may be of any suitable conventional construction such as, for example, that disclosed in US Pat. No. 2,990,710, issued July 4, 1961, to E. R. Burling, and essentially comprises an impeller 18, containing generally radially extending passageways communicating with the inlet conduit 14, which is rotatable driven by a drive motor to distribute the pulp-chemical mixture generally evenly over the cross section of the vessel 12. A vent pipe 26 is connected to the upper end of the vessel 12 for venting the latter in the conventional manner is provided with a gas scrubber 28 having a dilute solution of sodium hydroxide or other suitable liquid for preventing air pollution.

A plurality of outlets are tangentially connected to the vessel 12 for discharging bleached pulp or treated stock from the vessel 12 at locations spaced vertically from each other and spaced above the vessel lower end. In each embodiment of the invention, the elevations at which the outlets are connected to the vessel 12 and the number of vertically spaced outlets are preselected such that alternative employment of the outlets provides the desired or suitable alternative periods of retention of the pulp-chemical mixture in the vessel 12, Hence, although only two vertically spaced outlets are shown in the illustrated embodiments of the invention, it will be understood that this number of outlets has been shown for the purpose if illustration and alternatively further outlets could be tangentially connected to the vessels at additional vertically spaced locations.

As illustrated in FIG. I, a first outlet or stock discharge conduit 32 is tangentially connected to the vessel 12 at a location slightly above its longitudinal midpoint and a second outlet or stock discharge conduit 34 is tangentially connected to the vessel 12 adjacent the upper end thereof. The outlets 32, 34 are shown as provided with individual shutoff calves 36, 38, respectively; it will be understood, however, that, as the outlet 34 is connected adjacent the upper end of the vessel 12, the valve 38 could be omitted. The outlets 32, 34 are connected in parallel to a bleached pulp discharge conduit or manifold 40 which is provided with a pump 42 operable to pump the bleached pulp through the conduit 40 to a conventional pulp washer, chemical extractor, or other succeeding apparatus (not shown) in the paper making process. The discharge conduit 40, downstream of its connections to the outlets 32, 34, is provided with a control valve 44 shown schematically as operated by a conventional valve actuator 46 which may be controlled by a control 48 responsive to the level of mixture in the vessel 12 whereby the valve 44 is automatically adjusted to maintain the desired level of mixture in the vessel 12.

In accordance with the present invention, the apparatus 10 is provided with liquid supply means including at least one liquid supply conduit tangentially connected to the vessel 12 at a location vertically adjacent and closely spaced above each of the connections of the outlets 32, 34 to the vessel 12. More particularly as illustrated in FIG. 1, a liquid supply conduit 50 is tangentially connected to the vessel 12 at a location closely spaced above the connection of the outlet 32 to the vessel 12 and another liquid supply conduit 52 is tangentially connected to the vessel 12 at a location closely spaced above the connection of the outlet 34 thereto. The liquid supply conduits 50, 52 are connected in parallel to the discharge conduit 40 to receive therefrom a minor portion of the bleached pulp discharged from the vessel 12. The liquid supply conduits 50, 52 are provided with individual shutoff valves 54, 56, respectively, operable to permit alternative flow of bleached pulp through the conduits 50, 52 to the vessel 12. The valves 54, 56 may be operated by valve actuators 58, 60, respectively, and the latter each electrically or pneumatically connected to the one of the valve actuators 62, 64 operating the valve 36 or 38 in the outlet connected to the vessel 12 vertically thereadjacent. Moreover, the actuators 58, 62 operating the valve 54, 36 in the vertically adjacent conduit 50 and outlet 32 may be electrically or pneumatically connected to be operated by a single control 66 such that the discharge of bleached pulp through the outlet 32 and the supply of bleached pulp through the liquid supply conduit 50 are selectively concurrently initiated and stopped by operation of the control 66. Similarly, the actuators 60, 64 operating the valves 56, 38 in the vertically adjacent supply conduit 52 and outlet 32 may be pneumatically or electrically interconnected and controlled by a single control 68 to provide concurrent opening and closing of the valves 56, 38.

In the operation of the apparatus 10, the controls 66, 68 are operated to permit discharge of bleached pulp through the outlet 32 or 34 at a location to provide suitable retention of the pulp-chemical mixture in the vessel 12 and simultaneous supply of bleached pulp through the liquid supply conduit 50 or 52 connected to the vessel 12 vertically adjacent the outlet through which the bleached pulp is discharged. Thus, assuming for example that bleached pulp is to be discharged through the outlet 32, the control 66 is operated to cause the actuators 58, 62 to open the valves 54, 36. The control 68, conversely, is operated to cause the valves 56, 38 to be closed. The valve 44, as aforementioned, controls the normal operating level of mixture in the vessel 12; and such operating level is maintained slightly vertically above the outlet 32.

Pulp-chemical mixture from the source 24 is continuously pumped at a controlled volume and rate by the pump 22 through the inlet conduit 14 and stock distributor 16 to the lower end of the vessel 12, whereupon it moves upwardly in the vessel 12 generally evenly distributed over the vessel cross section. This upward movement of the mixture has a generally parabolic velocity profile as the portion of the mixture adjacent the vessel wall takes longer to attain the level of the outlet 32 than does the mixture portion adjacent the center of the vessel 12. Simultaneously with such upflow, however, a minor portion of discharged bleached pulp is continuously supplied to the vessel 12 through the conduit 50 and swirls downwardly along the vessel wall towards the outlet 32. This downwardly flowing pulp provides a liquid pad or layer above the level of the outlet 32 to thereby prevent air contamination of the upwardly moving the mixture and also acts to substantially equalize the retention period of mixture in the vessel 12 by gradually discharging or drawing off through the outlet 32 bleached pulp from the periphery of the upwardly moving mixture,

Operation of the apparatus 10 with bleached pulp discharged through the outlet 34 is attained by actuating the controls 66, 68 to close the valves 54, 36 and open the valves 56, 38. Thereafter, such operation proceeds in a manner believed to be apparent from the aforegoing description and with a minor portion of the bleached pulp returned to the vessel 12 through the conduit 52.

FIG. 3, wherein parts corresponding to those aforedescribed with regard to the embodiment of the invention illustrated in FIGS. 1 and 2 are designated by the reference character for the corresponding aforedescribed part followed by the suffix 21, illustrates a stock treatment apparatus in the form of a pulp-bleaching apparatus 10a constructed in accordance with an alternative embodiment of the invention. The apparatus 10a primarily differs from the aforedescribed apparatus 10 in that, rather than being constructed to supply bleached pulp above the upwardly moving mixture in the vessel, it is particularly adapted for so supplying a liquid or chemical extracted from the bleached pulp.

More particularly, as illustrated in FIG. 3, the bleached pulp discharge conduit or manifold 40a is connected to supply the bleached pulp to a chemical extractor, designated generally as 70, which may be of any conventional construction suitable for removing liquid or chemical from bleached pulp. The extractor 70 discharges pulp to a conduit 72 through which the pulp is pumped by a pump 74 to the succeeding process of the paper making operation. The extractor 70 discharges the removed chemical or liquid through a conduit 76 to a vat or reservoir 78 in which the separated liquid may be diluted. The reservoir 78 is connected to a liquid discharge conduit 80 provided with a pump 82, and communicates through the conduit 80 with the supply conduit 84 feeding the liquid supply conduits 50a, 52a. The conduit 80, as will be noted, is connected to the discharge conduit 40a for supplying thereto diluted liquid in excess of that required in the liquid supply conduits 50a, 52a, and a valve 86 is interposed in the conduit 80 intermediate its connections to the conduits 84, 40a. The valve 86 may be operated by an actuator 88 under the control of a control 90, in turn, connected to the extractor 70 to be controlled thereby. Also, the flow of bleached pulp through the discharge conduit 40a may be controlled by a valve 92 operated by an actuator 94 under the control of a control 96 responsive to the level of the mixture in the vessel 12.

The operation of the apparatus a proceeds in the manner aforedescribed with regard to the apparatus 10 except that the diluted extracted chemical is alternatively supplied to the vessel 12a through the liquid supply conduits 50a, 52a.

Although the illustrated embodiments of the invention disclose the employment of either bleached pulp or a diluted chemical or liquid extracted from bleached pulp as the liquid supplied to the vessel, it will be seen that the scope and spirit of the invention are not so limited; and alternatively other suitable liquids could be used.

In the practice of the method provided by the present invention, stock may be treated (preferably pulp bleached) in a generally upright elongated reaction vessel provided with tangential outlets at vertically spaced locations along its length, through the steps comprising continuously introducing inlet stock (in the instance of pulp bleaching, a mixture of pulp and bleaching chemical) to the vessel at the lower end thereof whereby the stock is generally evenly distributed across the vessel cross section and moves upwardly in the vessel towards the upper end thereof, continuously tangentially discharging treated stock (for example, bleached pulp) through the one of the outlets positioned at a location to provide a suitable period of retention of the stock in the vessel, and simultaneously with the discharge of treated stock through each outlet continuously tangentially supplying liquid to the vessel at a location vertically adjacent and closely spaced above the one of the outlets through which the treated stock is discharged whereby during the discharge through each outlet the liquid is supplied to the vessel at a location vertically adjacent and closely spaced above such outlet. The liquid supplied to the vessel may be the treated stock or bleached pulp discharged from the vessel, a liquid separated from the treated stock or bleached pulp discharged from the vessel, or any other suitable liquid.

From the aforegoing it will be seen that the invention provides a new and improved means and method for accomplishing all of the aforementioned objects and advantages. It will be understood, however, that, although only two embodiments of the invention have been illustrated and hereinbefore specifically described, the invention is not limited merely to these embodiments but rather contemplates other embodiments and variations employing the disclosed concepts and teachings.

Having thus described my invention, I claim:

1. Stock treatment apparatus comprising a generally upright elongated reaction vessel, said reaction vessel being provided with inlet means for supplying inlet stock to one end of said vessel whereupon the inlet stock moves longitudinally through said vessel towards the other end thereof, a plurality of stock discharge conduits open to said vessel for discharging treated stock from said vessel at locations spaced vertically from each other and spaced vertically from said one end of said vessel, and liquid supply means for supplying liquid to said vessel at a location vertically adjacent each of said locations at which treated stock is discharged from said vessel by said stock discharge conduits, said stock discharge conduits being alternatively operable for discharging treated stock from said vessel and said liquid supply means being operable during discharge of treated stock through each of said stock discharge conduits for supplying liquid to the vessel at the location vertically adjacent that from which the treated stock is discharged.

2. Stock treatment apparatus according to claim 1, further comprising a manifold conduit, said liquid supply means comprising a plurality of liquid supply conduits individually connected to both said manifold conduit and said vessel.

3. Stock treatment apparatus according to claim 2, wherein said inlet means is arranged to supply the inlet stock to the lower end of said vessel, and said liquid supply conduits are each connected to said vessel above the connection of a respective one of said stock discharge conduits to said vessel.

4. Stock treatment apparatus according to claim 1, further comprising a manifold discharge conduit connected to said stock discharge conduits to receive treated stock therefrom, said liquid supply means comprising a plurality of liquid supply conduits individually connected to said manifold discharge conduit and to said vessel for separately supplying treated stock from said manifold discharge conduit to said vessel.

5. Stock treatment apparatus comprising a generally upright elongated reaction vessel, said reaction vessel being provided with inlet means for continuously supplying inlet stock to the lower end of said vessel whereby the inlet stock moves upwardly in said vessel towards the upper end thereof, a plurality of stock discharge conduits tangentially connected to said vessel at locations spaced vertically from each other and spaced above the lower end of said vessel, said stock discharge conduits being open to said vessel for discharging treated stock therefrom, a plurality of liquid supply conduits for supplying liquid to said vessel, said liquid supply conduits including a liquid supply conduit connected to said vessel vertically adjacent and above each of the connections of said stock discharge conduits to said vessel, and valve means for controlling the discharge of treated stock through said stock discharge conduits and the supply of liquid through said liquid supply conduits, said valve means being operable for permitting alternative discharge of treated stock through said stock discharges conduits and simultaneous supply of liquid through the respective liquid supply conduit connected to said vessel vertically adjacent the stock discharge conduit discharging from said vessel whereby during discharge through each stock discharge conduit liquid is supplied to said vessel vertically adjacent and above the stock discharge conduit.

6. Stock treatment apparatus according to claim 5, further comprising a manifold conduit, said liquid supply conduits being tangentially connected to said vessel and individually connected to said manifold conduit to receive liquid therefrom.

7. Stock treatment apparatus according to claim 6, wherein said manifold conduit is connected to said stock discharge conduits to receive treated stock therefrom whereby the liquid supplied to said vessel by said liquid supply conduits is treated stock discharged through said stock discharge conduits.

8. Stock treatment apparatus according to claim 6, further comprising reservoir means for containing liquid other than treated stock, said manifold conduit being connected to said reservoir means to receive liquid therefrom.

9. A pulp-bleaching apparatus comprising a generally vertical elongated reaction vessel, inlet means for supplying a mixture of pulp and bleaching chemical to said vessel at the lower end thereof whereby the mixture moves upwardly in said vessel towards the upper end thereof, a first pulp discharge conduit tangentially connected to said vessel at a location spaced above the lower end of said vessel, a second pulp discharge conduit tangentially connected to said vessel at a location spaced above said connection of said first pulp discharge conduit to said vessel, said pulp discharge conduits being open to said vessel to discharge bleached pulp from said vessel at the locations of their said connections to said vessel, first liquid supply means tangentially connected to said vessel vertically adjacent and above said connection of said first pulp discharge conduit to said vessel, second liquid supply means tangentially connected to said vessel vertically adjacent and above said connection of said second pulp discharge conduit to said vessel, a manifold conduit separately connected to each of said liquid supply means for supplying liquid thereto, and valve means for controlling discharge of bleached pulp through said pulp discharge conduits and supply of liquid through said liquid supply means, said valve means being operable for permitting alternative discharge through said pulp discharge conduits and simultaneously supply of liquid through the liquid supply means connected to said vessel vertically adjacent the pulp discharge conduit discharging bleached pulp from said vessel whereby during discharge through each pulp discharge conduit liquid is tangentially supplied to said vessel vertically adjacent and above the pulp discharge conduit.

10. A pulp-bleaching apparatus according to claim 9, wherein said second pulp discharge conduit is connected to said vessel adjacent the upper end thereof, and said inlet means includes a stock distributor in said vessel at the lower end thereof for distributing the pulp-chemical mixture over the cross section of said vessel.

11. A pulp-bleaching apparatus according to claim 10, wherein said pulp discharge conduits are individually connected to said manifold conduit for discharging bleached pulp thereto whereby the liquid supplied by said manifold conduit to said liquid supply means is bleached pulp discharged through said pulp discharge conduits.

12. A pulp-bleaching apparatus according to claim 10, further comprising reservoir means for containing liquid separated from bleached pulp discharged through said pulp discharge conduits, said manifold conduit being connected to said reservoir means to receive the liquid therefrom.

13. A method for treating stock in a generally upright elongated reaction vessel provided with a plurality of outlets at 10- cations spaced vertically along its length, comprising the steps of introducing inlet stock to one end of said vessel whereupon the inlet stock moves longitudinally through said vessel towards the other end thereof, and discharging treated stock through said outlets alternatively while simultaneously during the discharge of treated stock through each outlet supplying liquid to said vessel at a location vertically adjacent the one of said outlets through which the treated stock is discharged.

14. A stock treatment method according to claim 13, wherein the inlet stock is supplied to said vessel at the lower end thereof and moves upwardly in said vessel towards the upper end thereof.

15. A method for bleaching pulp in a generally upright elongated reaction vessel provided with tangential outlets at vertically spaced locations along its length,'comprising the steps of continuously introducing a mixture of pulp and bleaching chemical to said vessel at the lower end thereof whereby the mixture moves upwardly in said vessel towards the upper end thereof, continuously discharging bleached pulp through the one of said outlets positioned at a location to provide a suitable period of retention of the mixture in said vessel, and simultaneously with the discharge of bleached pulp through each outlet continuously supplying liquid to said vessel at a location vertically adjacent and above the one of said outlets through which the bleached pulp is discharged whereby during the discharge of bleached pulp through each outlet the liquid is supplied to said vessel at a location vertically adjacent and above such outlet.

16. A pulp-bleaching method according to claim 15, wherein said liquid is tangentially supplied to said vessel.

17. A pulp-bleaching method according to claim 16, wherein the supplied liquid is bleached pulp discharged from said vessel.

18. A pulp-bleaching method according to claim 16, wherein the supplied liquid includes a liquid separated from bleached pulp discharged from said vessel.

19. A pulp-bleaching apparatus according to claim 9, wherein said valve means comprises a first valve for controlling discharge of pulp through said first pulp discharge conduit and a second valve for controlling supply of liquid through said first liquid supply means, and further comprising single control means for operating both of said valves.

20. A stock treatment method according to claim 14, wherein said liquid is tangentially supplied above the outlets.

21. A stock treatment method according to claim 20, wherein the supplied liquid is treated stock discharged from the vessel.

22. A stock treatment method according to claim 20,

wherein the supplied liquid comprises a liquid separated from treated stock discharged from the vessel. 

2. Stock treatment apparatus according to claim 1, further comprising a manifold conduit, said liquid supply means comprising a plurality of liquid supply conduits individually connected to both said manifold conduit and said vessel.
 3. Stock treatment apparatus according to claim 2, wherein said inlet means is arranged to supply the inlet stock to the lower end of said vessel, and said liquid supply conduits are each connected to said vessel above the connection of a respective one of said stock discharge conduits to said vessel.
 4. Stock treatment apparatus according to claim 1, further comprising a manifold discharge conduit connected to said stock discharge conduits to receive treated stock therefrom, said liquid supply means comprising a plurality of liquid supply conduits individually connected to said manifold discharge conduit and to said vessel for separately supplying treated stock from said manifold discharge conduit to said vessel.
 5. Stock treatment apparatus comprising a generally upright elongated reaction vessel, said reaction vessel being provided with inlet means for continuously supplying inlet stock to the lower end of said vessel whereby the inlet stock moves upwardly in said vessel towards the upper end thereof, a plurality of stock discharge conduits tangentially connected to said vessel at locations spaced vertically from each other and spaced above the lower end of said vessel, said stock discharge conduits being open to said vessel for discharging treated stock therefrom, a plurality of liquid supply conduits for supplying liquid to said vessel, said liquid supply conduits including a liquid supply conduit connected to said vessel vertically adjacent and above each of the connections of said stock discharge conduits to said vessel, and valve means for controlling the discharge of treated stock through said stock discharge conduits and the supply of liquid through said liquid supply conduits, said valve means being operable for permitting alternative discharge of treated stock through said stock discharge conduits and simultaneous supply of liquid through the respective liquid supply conduit connected to said vessel vertically adjacent the stock discharge conduit discharging from said vessel whereby during discharge through each stock discharge conduit liquid is supplied to said vessel vertically adjacent and above the stock discharge conduit.
 6. Stock treatment apparatus according to claim 5, further comprising a manifold conduit, said liquid supply conduits being tangentially connected to said vessel and individually connected to said manifold conduit to receive liquid therefrom.
 7. Stock treatment apparatus according to claim 6, wherein said manifold conduit is connected to said stock discharge conduits to receive treated stock therefrom whereby the liquid supplied to said vessel by said liquid supply conduits is treated stock discharged through said stock discharge conduits.
 8. Stock treatment apparatus according to claim 6, further comprising reservoir means for containing liquid other than treated stock, said manifold conduit being connected to said reservoir means to receive liquid therefrom.
 9. A pulp-bleaching apparatus comprising a generally vertical elongated reaction vessel, inlet means for supplying a mixture of pulp and bleaching chemical to said vessel at the lower end thereof whereby the mixture moves upwardly in said vessel towards the upper end thereof, a first pulp discharge conduit tangentially connected to said vessel at a location spaced above the lower end of said vessel, a second pulp discharge conduit tangentially connected to said vessel at a location spaced above said connection of said first pulp discharge conduit to said vessel, said pulp discharge conduits being open to said vessel to discharge bleached pulp from said vessel at the locations of their said connections to said vessel, first liquid supply means tangentially connected to said vessel vertically adjacent and above said connection of said first pulp discharge conduit to said vessel, second liquid supply means tangentially connected to said vessel vertically adjacent and above said connection of said second pulp discharge conduit to said vessel, a manifold conduit separately connected to each of said liquid supply means for supplying liquid thereto, and valve means for controlling discharge of bleached pulp through said pulp discharge conduits and supply of liquid through said liquid supply means, said valve means being operable for permitting alternative discharge through said pulp discharge conduits and simultaneous supply of liquid through the liquid supply means connected to said vessel vertically adjacent the pulp discharge conduit discharging bleached pulp from said vessel whereby during discharge through each pulp discharge conduit liquid is tangentially supplied to said vessel vertically adjacent and above the pulp discharge conduit.
 10. A pulp-bleaching apparatus according to claim 9, wherein said second pulp discharge conduit is connected to said vessel adjacent the upper end thereof, and said inlet means includes a stock distributor in said vessel at the lower end thereof for distributing the pulp-chemical mixture over the cross section of said vessel.
 11. A pulp-bleaching apparatus according to claim 10, wherein said pulp discharge conduits are individually connected to said manifold conduit for discharging bleached pulp thereto whereby the liquid supplied by said manifold conduit to said liquid supply means is bleached pulp discharged through said pulp discharge conduits.
 12. A pulp-bleaching apparatus according to claim 10, further comprising reservoir means for containing liquid separated from bleached pulp discharged through said pulp diScharge conduits, said manifold conduit being connected to said reservoir means to receive the liquid therefrom.
 13. A method for treating stock in a generally upright elongated reaction vessel provided with a plurality of outlets at locations spaced vertically along its length, comprising the steps of introducing inlet stock to one end of said vessel whereupon the inlet stock moves longitudinally through said vessel towards the other end thereof, and discharging treated stock through said outlets alternatively while simultaneously during the discharge of treated stock through each outlet supplying liquid to said vessel at a location vertically adjacent the one of said outlets through which the treated stock is discharged.
 14. A stock treatment method according to claim 13, wherein the inlet stock is supplied to said vessel at the lower end thereof and moves upwardly in said vessel towards the upper end thereof.
 15. A method for bleaching pulp in a generally upright elongated reaction vessel provided with tangential outlets at vertically spaced locations along its length, comprising the steps of continuously introducing a mixture of pulp and bleaching chemical to said vessel at the lower end thereof whereby the mixture moves upwardly in said vessel towards the upper end thereof, continuously discharging bleached pulp through the one of said outlets positioned at a location to provide a suitable period of retention of the mixture in said vessel, and simultaneously with the discharge of bleached pulp through each outlet continuously supplying liquid to said vessel at a location vertically adjacent and above the one of said outlets through which the bleached pulp is discharged whereby during the discharge of bleached pulp through each outlet the liquid is supplied to said vessel at a location vertically adjacent and above such outlet.
 16. A pulp-bleaching method according to claim 15, wherein said liquid is tangentially supplied to said vessel.
 17. A pulp-bleaching method according to claim 16, wherein the supplied liquid is bleached pulp discharged from said vessel.
 18. A pulp-bleaching method according to claim 16, wherein the supplied liquid includes a liquid separated from bleached pulp discharged from said vessel.
 19. A pulp-bleaching apparatus according to claim 9, wherein said valve means comprises a first valve for controlling discharge of pulp through said first pulp discharge conduit and a second valve for controlling supply of liquid through said first liquid supply means, and further comprising single control means for operating both of said valves.
 20. A stock treatment method according to claim 14, wherein said liquid is tangentially supplied above the outlets.
 21. A stock treatment method according to claim 20, wherein the supplied liquid is treated stock discharged from the vessel.
 22. A stock treatment method according to claim 20, wherein the supplied liquid comprises a liquid separated from treated stock discharged from the vessel. 